There is a case where as a means to increase voltage of a power conversion device, a circuit configuration in which a plurality of semiconductor switching elements (e.g. IGBTs) are connected in series is applied. The switching element is brought into ON/OFF states by ON/OFF of a gate command inputted to a gate terminal of the switching element.
When the gate commands of the switching elements connected in series are simultaneously changed to OFF, breakdown of the switching element might occur due to a difference of agate command timing and/or a difference in voltage between the switching elements during a tail current period.
Patent Document 1 has proposed a technique of equalizing a collector-emitter voltage (a voltage between the collector and an emitter) of the switching element connected in series by applying, between a collector terminal and a gate terminal of the switching element, a protection circuit configured by a resistance and a capacitor. FIG. 3 shows a circuit configuration proposed in Patent Document 1.
In FIG. 3, in an IGBT module 15, an IGBT 17 and a diode 18 are connected parallel in reverse, and a collector terminal C, an emitter terminal E, a gate terminal G and a control emitter terminal E1 are provided. The IGBT 17 becomes ON or OFF on the basis of a gate voltage Vg applied across terminals G and E through a resistance 21, and a state between C-E terminals of the IGBT module 15 is brought into an ON state or an OFF state, thereby performing a switching operation. Here, a reference sign 19 denotes a gate capacitance of the IGBT module 15, and reference signs 20 and 20a denote floating inductances (or stray inductances) appearing in a wiring inside the IGBT module 15.
A resistance 23 and a capacitor 22, as the protection circuit, are connected in series between the collector terminal C and the gate terminal G of the IGBT 17.
Further, Patent Documents 3 and 4 have proposed a technique of applying a protection circuit in which an element having avalanche characteristics and a capacitor are combined. In addition, Patent Documents 5 and 6 have proposed a protection circuit in which an avalanche element, a resistance and a capacitor are combined.
FIG. 4 shows a circuit configuration proposed in Patent Document 5. In FIG. 4, an inductive load L whose one end is connected to a DC voltage source +V is connected, at the other end thereof, to a drain Dr of a field-effect transistor T. Between the drain Dr and a gate G of the field-effect transistor T, a diode D whose polarity is as shown in the drawing, avalanche elements ZD3˜ZD10 whose polarities are opposite to that of the diode D and a resistance Rs are sequentially connected in series. A capacitor C1 is connected parallel to the avalanche element ZD3, and a resistance R3 and a capacitor C3 are connected in series between both ends of the avalanche element ZD5. A resistance R5 and a capacitor C5 are connected parallel between both ends of the avalanche element ZD7, and a resistance R8 is connected parallel to the avalanche element ZD10.
Meanwhile, Non-Patent Document 1 has disclosed a technique of protecting a switching element from overvoltage breakdown between a gate and an emitter of the switching element by connecting a Zener diode between the gate and the emitter of the switching element. FIG. 5 shows a circuit configuration disclosed in Non-Patent Document 1.
In FIG. 5, a Zener diode 31 and a diode Ds whose polarity is as shown in the drawing are connected in series between a collector and a gate of an IGBT 50. A Zener diode 32 and a Zener diode 33 are connected in series with their polarities being opposite to each other between the gate and an emitter of the IGBT 50. Further, a gate resistance RGoff and a drive voltage source 34 are connected in series between the gate and the emitter of the IGBT 50. Lk denotes an inductance.
By combining circuits of FIGS. 4 and 5, it is possible to configure a circuit having both functions of overvoltage protection of a collector-emitter voltage of the switching element and overvoltage protection of a gate-emitter voltage of the switching element, as shown in FIG. 6.
In FIG. 6, a reference sign 50 denotes an IGBT as the switching element, which indicates one of a plurality of the IGBTs that are connected in series in a power conversion device. A reference sign 60 is a drive IC that outputs a gate command (a voltage of ON/OFF command) to a gate of the IGBT 50 through a gate resistance Rg.
Between a collector and a gate of the IGBT 50, avalanche elements (avalanche diodes having avalanche characteristics) D1˜D5 whose polarities are as shown in the drawing and a resistance R4 are sequentially connected in series. A capacitor C1 and a resistance R1 are connected parallel between both ends of the avalanche element D4, and a capacitor C2 and a resistance R2 are connected parallel between both ends of the avalanche element D5. A Vice overvoltage protection circuit 41 is configured by these avalanche elements D1˜D5, capacitors C1 and C2 and resistances R1, R2 and R4.
A cathode of a Zener diode ZD11 is connected to a common connection point of the gate of the IGBT 50 and the resistance R4. An anode of the Zener diode ZD11 is connected to an emitter of the IGBT 50 through an anode and a cathode of a Zener diode ZD12. A Vge overvoltage protection circuit 42 is configured by these Zener diodes ZD11 and ZD12.